1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) projection system, and more particularly, to a device for enhancing contrast of an image which is projected on a screen according to a result of comparing an average level of an image signal and a reference level.
2. Description of the Related Art
In general, a projection system adopting a liquid crystal display (LCD) scans an image signal which is supplied from an image driver and displayed on a LCD panel with light incident from an illumination optical system. Here, the scanned light is projected on the screen through a deflection plate and a projection lens, to thereby regenerate an enlarged image.
A conventional LCD projection system will be described with reference to FIG. 1.
The LCD projection system of FIG. 1 includes illumination optical system 10 through 14, an image driver 16, a LCD panel 15 and a projection optical system 17 through 19.
The illumination optical system includes an optical unit 10 for emitting and reflecting light, and a relay lens 11 for making an optical feature of the light emitted from the optical unit 10 and reducing an angle of divergence, by use of a total reflection feature. Also, the illumination optical system includes a collimating lens 12 for converting the light passed through the relay lens 11 into parallel light, a dichroic mirror 13 for separating the parallel light into three primary color signal components R, G and B and altering a light path, and a first polarizing plate 14 for polarizing each color signal component supplied from the dichroic mirror 13 to be incident to the LCD panel 15.
The image driver 16 performs a predetermined signal processing with respect to an image signal input from an image signal processor (not shown) to be displayed on the LCD panel 15, and converts the image signal into a driving control signal to be supplied to the LCD panel 15. Here, the image driver 16 further performs an ABL (auto brightness limiter) control function which is well known to a person skilled in the art. The ABL control function is to obtain an average level of an image signal to be displayed on the LCD panel 15, or an externally input image signal, so as to maintain the brightness of the image to be displayed on the LCD panel 15 consistently. Then, the average level is compared with a set reference level, and a gain of the image signal supplied to the LCD panel 15 is adjusted according to the comparison result. That is, if the obtained average level is higher than the reference level through the ABL control function, a low-level ABL control signal for decreasing the gain of the image signal supplied to the LCD panel 15 is applied to an amplifier (not shown). Meanwhile, if the former is lower than the latter, a high-level ABL control signal for increasing the gain of the image signal supplied to the LCD panel 15 is applied to the amplifier. The above control operation is linearly performed.
The LCD panel 15 displays a corresponding image thereon according to the driving control signal of the image signal supplied from the image driver 16 and scans the image according to the intensity of the illuminated light incident from the illumination optical system.
A second polarizing plate 17 disposed between the LCD panel 15 and a focusing lens 18 selectively transmits or blocks the light scanned from the LCD panel 15, according to the polarizing component of the light. The focusing lens 18 focuses the light transmitted through the second polarizing plate 17 and transfers the focused light to a projection lens 19. The projection lens 19 projects the light transferred from the focusing lens 18 onto a screen (not shown), to thereby enlarge the image regenerated on the screen.
However, in the case of the conventional LCD projection system, the contrast of the LCD panel itself on which the image signal is primarily displayed is much lower than that of a display device such as a cathode ray tube (CRT), and the polarizing plate disposed between the LCD panel and the projection lens does not block the light completely although the degree of polarization is 99.5% or more. Thus, since the black level of the image having passed through the LCD panel and the polarizing plate and regenerated on the screen does not become a perfect black level, the conventional LCD projection system has a problem in which the contrast of the regenerated image is greatly lowered.